Abstract: The composite index gas is less affected by environmental factors and it could timely and accurately predict the coal spontaneous combustion process. In this paper, by using the constant temperature oxidation device, the heat storage environment of the goaf was simulated to study the generation characteristics of the index gas in the oxidation reaction time of the residual coal in goaf. The experiments were conducted with different sizes of coal samples in Guandi Coal Mine. Results of experiment show that the temperature increases, the specific values of O₂/(CO+CO₂ )and H₂/CH₄ decrease, the specific value of CO/CO₂ increases, when the temperature is over 80 ℃, the specific value will change dramatically. With the decrease of particle size, the specific values of H₂/CH₄ and CO/CO₂ increase, and the specific value of O₂/(CO+CO₂ )decreases. The smaller the particle size is, the more obvious the trend is. When the coal sample is heated at 150 ℃ for about 60 minutes, the change rate of the composite index gas ratio is greatly.